Coating compositions and processes

ABSTRACT

A process is provided for preparing a hexavalent chromium containing emulsion coating composition without causing coagulation of the emulsion and for providing an emulsion coating composition containing at least one water insoluble particulate film-forming thermoplastic resin which is highly effective as a coating for a metal substrate to give new and improved results in the preparation of resin coated metals, particularly steel, aluminum, magnesium, and zinc surfaced articles including galvanized iron or steel.

CROSS REFERENCE TO RELATED CASES

This application is a division of my application Ser. No. 679,694 filedApr. 23, 1976, now U.S. Pat. No. 4,069,187, which in turn is acontinuation of my application Ser. No. 505,212, filed Sept. 12, 1974,now abandoned.

BACKGROUND

It is well known in the metal coating industry that there is acontinuing need for coatings used for paint bonding and corrosionresistance. This is especially true where the metal is steel, aluminum,magnesium, aluminum alloys and zinc surfaced articles includinggalvanized iron or steel, where such coatings are required in order toprotect the articles against deterioration.

In the past many such coatings have been suggested and used such asphosphate, zincate and anodized films but, in general, these coatingshave left much to be desired because of various deficiencies includingcomplex and often difficult operating procedures and high costs.

Chromate conversion coatings have been used with varying degrees ofsuccess. The effective protective ingredient in these coatings seems tobe chromium in the hexavalent state. It has long been recognized that itwould be desirable to combine in a single coating composition an organicresinous film forming component which is water insoluble and chromium ina hexavalent state. In particular it would be desirable to prepare anemulsion coating composition containing the resinous organicfilm-forming component in the discontinuous phase and water in thecontinuous phase, together with chromium in a hexavalent state. If thiscould be done, latex emulsion paints such as, for example, acrylicemulsion paints, could be applied to metal substrates and a singlecoating would give a combined beneficial effect due to the presence ofthe chromium in the hexavalent state as well as the film-formingprotective properties of the organic resin. Unfortunately, attempts toaccomplish this result have not been successful primarily for the reasonthat the addition of the chromium in a hexavalent state by the usualmeans employing potassium dirhromate sodium dichromate, ammoniumdichromate, (and their respective chromates) or chromic acid causescoagulation of the emulsion, produces large amounts of sediment, anddestroys its capability of being applied as a uniform coating.

OBJECTS

One of the objects of the present invention is to provide a new andimproved process for incorporating chromium in a hexavalent state intoan emulsion coating composition of the type in which the discontinuousphase contains at least one water insoluble particulate film-formingorganic thermoplastic resin and the continuous phase contains water byadding to such emulsion a hexavalent chromium compound in such a waythat coagulation of the emulsion does not occur.

Another object of the invention is to produce new and improvedhexavalent chromium containing emulsion coating compositions of the typedescribed which are stable and have a substantial shelf life.

Still a further object of the invention is to provide a new and usefultype of water insoluble pigment which is especially useful in thepreparation of emulsion coating compositions of the type described.

Another object of the invention is to provide a process of coatingmetals especially steel, aluminum, magnesium and zinc surface metalsincluding galvanized iron or steel wherein the applied coating not onlyenhance corrosion resistance but also when used as primer coatings willadhere to subsequently applied finishing coatings. Other objects willappear hereinafter.

BRIEF SUMMARY OF THE INVENTION

In accordance with the invention a process is provided for preparing ahexavalent chromium containing emulsion coating composition withoutcausing coagulation of the emulsion and for producing an emulsioncoating composition containing chromium in the hexavalent state and atleast one water insoluble particulate film-forming organic thermoplasticresin by mixing a body of an emulsion coating composition containingwater in the continuous phase and in the discontinuous phase at leastone water insoluble particulate film-forming organic thermoplastic resinwith a gradually added dilute aqueous solution of an inorganic ionizablewater soluble hexavalent chromium compound having a concentration of 1%to 10% by weight, calculated as Cr, while cooling and maintaining a pHwithin the range of 2 to 10.5 and maintaining said body in a non-foamingstate. The process is preferably carried out by causing the body ofemulsion to rotate and gradually adding the dilute dolution to aperipheral portion of the rotating body.

As a further feature of the invention, if it is desired to incorporate awater insoluble pigment into the emulsion coating composition, thethermoplastic resin used in forming the emulsion is divided into twoparts, the first part being emulsified in water and mixed with saidaqueous solution of said water soluble hexavalent chromium compound inthe manner previously described, and the second part being mixed withsaid pigment in sufficient proportions to coat said pigment, andthereafter combining said first and second parts.

Another embodiment of the invention involves the use of a particulartype of pigment and a process of making them, said pigment consistingessentially of CaSiF₆ and 0% to 10% by weight, calculated as Cr, ofhexavalent chromium added as an inorganic ionizable water solublehexavalent chromium compound.

The invention also involves the provision of a process for preparing apigment of the type previously described in which an inorganic calciumcompound and an inorganic silicofluoride are mixed in equimolecularproportions in sufficient water to form a thick slurry, the resultantmixture is neutralized and thereafter dried and ground.

In addition, the invention involves the use of the aforesaid emulsioncoating compositions under controlled conditions for coating a metalsubstrate, more particularly, steel, aluminum, magnesium, and zincsurfaced metals including galvanized iron or steel, wherein the appliedcoatings not only enhance corrosion resistance but also, when used asprimer coatings, will adhere to subsequently applied finishing coatings.

DETAILED DESCRIPTION OF THE INVENTION

Coating compositions which comprise water as the continuous phase andcontain in the discontinuous phase at least one water insolubleparticulate film-forming organic thermoplastic resin, with or withoutthermosetting resins, are well known in the art. Compositions of thistype have been widely used for painting or coating metal and for manypurposes they are quite satisfactory. In general, however, they leavemuch to be desired from the standpoint of corrosion inhibition and theability to withstand salt spray and exposure to boiling water. Generallyspeaking, these compositions deteriorate when exposed to salt spray andsuffer from the disadvantage that after being dried out or cured, whenapplied as a coating to a metal they will reabsorb moisture in thepresence of heat. This in turn causes peeling, and peeling will occur,for example, when a metal coated with one of these compositions isalternately exposed to the sun's rays followed by a rain storm. As aresult, so far as is known no practical method has been devised forproviding highly corrosion resistant coatings which are also resistantto the reabsorption of water when such coatings have been derived fromconventional pigmented or unpigmented emulsion type coating compositionsof the kind previously described.

Although it is known that hexavalent chromium when applied as a coatingto metal has a corrosion inhibiting effect, chromium in the hexavalentstate as, for example, in the form of sodium dichromate potassiumdichromate, ammonium dichromate, (or their respective chromate salts) orchromic acid, when mixed in a conventional manner with an emulsion ofthe type described causes coagulation thereby making the resultantemulsion unfit for use as a composition for coating and forming a filmon a metallic substrate.

This tendency to coagulate such emulsions is apparently due to thestrong oxidizing effect of the aforesaid chromium compounds not onlyupon resinous components of the emulsion but also upon other componentsthat are normally present in such emulsions such as, for example,alcohols and glycols. The reaction between these hexavalent chromiumcompounds and the aforesaid components of the emulsion is quite rapid ifthe chromium compounds are added either in a solid state or inconcentrated liquid form or if the emulsion is foaming.

It has been discovered in accordance with this invention, however, thatby using a dilute aqueous solution of an inorganic ionizable watersoluble hexavalent chromium compound having a concentration of 1% to 10%by weight and by adding such solution gradually to a peripheral portionof a body of the emulsion, or sections having rapid movement to disperseand dilute active chromium compound in the fastest possible manner,while agitating the emulsion in such a way as to rotate it, for example,by means of a centrally disposed paddle or other suitable meanseffective to produce non-violent agitation, it is possible toincorporate hexavalent chromium into an emulsion of the type describedwithout causing "shock" or coagulation of the emulsion. Inasmuch as thereaction between hexavalent chromium compounds and the components of theemulsion is exothermic and occurs rapidly, it is also desirable to coolthe emulsion while the dilute aqueous solution of the water solublehexavalent chromium compound is being added thereto. In addition, it isquite important to maintain the body of emulsion in a non-foaming statebecause the reaction of the hexavalent chromium compound with foamparticles is extremely rapid. Moreover, the pH of the emulsion should bemaintained within a range of 2 to 10.5. Preferably, the dilute aqueoussolution of the water soluble hexavalent chromium compound is addeddropwise to the main body of emulsion. It is also preferable to use asthe chromium compound either sodium dichromate, potassium dichromate,sodium chromate, potassium chromate (at least in part) or ammoniumdichromate or mixtures thereof. These compounds when dissolved in waterare acidic but not as acidic as chromic acid and therefore make thecontrol of the reaction simpler.

It is usually preferable to maintain a temperature in the emulsion whilecarrying out the aforesaid process within the range of 70° F. to 80° F.In general, the temperature should be above that giving ice crystalformation and not above 90° F. It is also preferable to rotate the bodyof the emulsion slowly, for example, 1 to 60 revolutions per minute. Thespeed of rotation will vary depending upon the size of the container.For example, a one liter container would be only six inches in diameterwhile a 55 gallon drum might be 30 inches in diameter and a largervessel of the type used in carrying out chemical reactions might be sixfeet or more in diameter. Inasmuch as the dilute solution of thehexavalent chromium compound is added to a peripheral portion orsections having the greatest movement of the rotating body of emulsion,the speed of rotation could be greater in a larger container.

The foregoing process is applicable to any emulsion coating compositioncontaining water as the continuous phase and in the discontinuous phaseone or more water insoluble particulate film-forming organicthermoplastic resins which are normally reactive with hexavalentchromium, with or without one or more thermosetting resins, which areadded to increase the impermeability of a coating when the coatingcomposition is applied to a substrate. A particular type ofthermoplastic resin emulsion is one containing 100% acrylic emulsionpolymer comprising 46±0.5% by weight solids having a pH of 9 to 11 andweighing approximately 8.9 pounds per gallon. Such an acrylic emulsionpolymer is available commercially under the name Rhoplex MV-1.

A preferred thermosetting resin for use in the practice of the inventionis an acrylic emulsion polymer containing 46±0.5% by weight solidshaving a pH of 9.5 to 10.5 and weighing approximately 9 pounds pergallon. Such a resin is available commercially under the name RhoplexAC-604. Examples of other suitable resins are those sold under the namesRhoplex B-5 and Rhoplex AC-33X.

While the foregoing resins are preferred in the practice of theinvention, the invention in its broadest aspects is not limited to theseparticular resins. The types of resins used are many, e.g., vinyl,polyester, epoxy and acrylic, either simple or modified, or mixed; crosslinking additives such as, for example, melamine resins, methylated ureaformaldehyde and synthetic type rubber type resins can be used.

Typical examples of resin combinations are thermoplastic (e.g., RhoplexMV-1)-thermosetting (e.g., Rhoplex AC-604); thermoplastic (e.g., RhoplexMV-1)-thermosetting (e.g., Rhoplex AC-604)-thermosetting (Epoxy PR-808);thermoplastic (e.g., Rhoplex MV-1)-thermosetting (e.g., RhoplexB-5)-thermosetting (e.g., Rhoplex E-1046) cross linking (e.g., UformiteM83, a melamine resin); and thermoplastic (e.g., RhoplexMV-1)-thermosetting (e.g., Rhoplex B-5)-thermosetting (e.g., RhoplexAC-33X and Rhoplex E-1046)-cross linking (e.g., HYCAR CTBN, UformiteM-83, and Beetle 65).

The various resins are formulated into emulsions in a conventionalmanner by mixing them in water with various additives includingdispersion agents, surfactants, deformers, coalescing agents, couplingagents, flow control agents, pH control agents and viscosity controlagents. The variety of these additives is quite large in number andwhile the end product produced will have somewhat different chemical andphysical characteristics depending upon the particular additives, exceptto the extent herein described, the invention is not limited toparticular additives. Examples of suitable coupling agents normallyemployed in making such emulsions are isopropyl alcohol, N-butylalcohol, and tertiary butyl alcohol. Examples of suitable coalescingagents are butyl Cellosolve, butyl carbitol and tributyl phosphate.Hydrolyzable esters and water soluble substances detract from shelf lifeand water and vapor insensitivity.

The preferred pH control materials are ammonium hydroxide,dimethylethanolamine, and diethanolamine. These substances are alkalineas contrasted with the aqueous solutions of the dichromates which areacidic. Tertiary amines such as dimethylethanolamine are preferred fromthe standpoint of enhancing the stability of the resultant emulsionswith which the hexavalent chromium compounds have been incorporated.Diethanolamine also gives good results. The use of ammonium hydroxide isalso effective but with the stability somewhat less. However, the use ofammonium hydroxide has the advantage that it decomposes more easilyafter the emulsion coating composition has been applied to a substrateand during the curing process. For this reason it is usually desirableto add the hexavalent chromium as ammonium dichromate. It is alsocustomary to use the pH control agents in the form of aqueous solutions,for example, ammonium hydroxide as a 28% aqueous solution and the aminesdiluted in equal weight proportions (i.e., 1:1) with water.

Examples of thickening agents are polyacrylic acid and salts thereof,for example, the sodium and ammonium salts such as Acrysol GS, AcrysolG-110 and Acrysol WS-24.

Examples of pigment dispersants, stabilizers and surfactants are thosematerials sold commercially under the names Tamol 731, Tamol 850, TritomCF-10 which is a water soluble nonionic oxyethylated benzyl ether ofoctyl phenol, Triton X-114 which is an oxyethylated octyl phenol, andEthoxylan E which is lanolin reacted with ethylene oxide.

Conventional defoaming agents can be employed as, for example, thesilicones and particular mention can be made of NOPCO NXZ, WITCO BALAB768.

Various types of pigments can be added to the emulsion coatingcompositions including lead silicate, chromate pigments (e.g., Oncor-M50which has a yellow brown color) and strontium chromate. However, for thepurpose of the present invention a particularly useful pigment is onewhich so far as is known has not heretofore been prepared and is notavailable commercially. This pigment is prepared by mixing equimolecularproportions of an inorganic calcium compound and an inorganicsilicofluoride in water sufficient to form a thick slurry, neutralizingthe resultant mixture, drying and grinding. The sources of calcium arepreferably calcium oxide, calcium hydroxide, calcium carbonate andcalcium bicarbonate. The sources of SiF₆ are preferably H₂ SiF₆ (26%solution in water), (NH₄)₂ SiF₆ and ZnSiF₆.

The reaction is exothermic and the mixing container should be jacketedand water cooled. In order to prepare a pigment containing hexavalentchromium, chromic acid is used in the neutralization in amounts up to10% by weight, calculated as Cr. If it is desired to prepare CaSiF₆without chromium, another inorganic acid, e.g., phosphoric acid, is usedas the neutralizing agent.

Excessive water can be removed from the completed product by allowingthe reaction mixture to stand for 24 hours, then decanting or extractingthe liquid surface layer. This material forms hard crystals which forthe purpose of the present invention should be reduced to a particlesize of 0.5 to 5 microns, either by ball milling or roller milling.Where the pigment contains hexavalent chromium its addition to anaqueous emulsion resin coating composition will enhance corrosioninhibition when the coating composition is applied to a metal substrate.

In general, emulsion coating compositions prepared in accordance withthe invention will have a solids content, excluding pigmentation, withinthe range of 15% to 35% by weight.

A given emulsion coating composition might be modified depending uponthe type of application, i.e., roll coating, spray coating, immersionand squeegee applications, electrostatic applications and other methodsof application.

Where the emulsion coating composition contains both thermoplasticresins and thermosetting resins the weight ratio of thermoplastic resinsto thermosetting resins is preferably within the range of 6:1 to 3:1,the ratio however, can be altered at will. The more thermosetting resinused the less the water and vapor sensitivity and extensibility.

If a cross linking agent is employed, the amount is preferably at least25% by weight of the resin to be cross linked.

Where pigments are added the weight ratio of pigment to resinous binderis usually within the range of 5:95 to 40:60, although it can be as lowas 1:99.

In the application of the coating compositions to a substrate such as ametal, those skilled in the art will recognize that cleaning andpreparation of the metal is quite important and may involve the usualcleaning methods, deoxidizing of the substrate, rinsing and drying.

After the emulsion coating composition is applied to a substrate, e.g.,steel, aluminum, magnesium, or a zinc surfaced substrate, it is dried orallowed to dry and cured, for example, at 400° F. for from 60 seconds tothree minutes or 600° F. from 30 to 90 seconds. Although the overalltemperature range of curing is 400° F. to 600° F. for periods of timefrom 30 seconds to 3 minutes, the time of heating is longer at lowertemperatures and shorter at the higher temperatures. Thetime-temperature relationship is preferably controlled to give a finalcoating having a pencil hardness of H to 2H.

The wet thickness of the applied coating is usually within the range of0.05 mil to 10 mils (a mil equalling 0.001 inch). A preferred thicknessof the wet coating is within the range of 0.1 to 0.3 mil. The greaterthe thickness of film the greater will be the difficulty of acquiring auniformly "cured" film.

The quality of the cured coating can be tested in a number of ways. Onesuch test is the salt spray test where a mist or fog of an aqueoussolution containing 5% sodium chloride or acetic acid acidified sodiumchloride is applied to coated panels over a predetermined time period.

Another type of test is the Q-Panel Humidity Panel Test in which thecoated panel is subjected to humdities of 90 to 100% at temperatures offrom 100° F. to 190° F. with drying in cyclical periods, the cyclesbeing timed at 0 to 60 minutes under high humdity conditions and 0 to 60minutes under drying conditions or in variation in 10 minute units oftime.

Another test which is perhaps the most strenuous of all involves boilingthe coated panels in distilled water for 30 minutes, 60 minutes or 120minutes. The area being tested may or may not be impacted or bent.Usually a pencil hardness test is made before and after the test(allowing 5 to 15 minutes recovery at room temperature) Observation canbe made for resistance to solvation of coating, chalking of coating andwater absorption. A good film or coating should be capable of returningto its original condition after this test.

Other tests can be conducted to determine resistance to over-bake cureusing pencil hardness, impact tests and bend tests. Continuity of filmcan be determined by using the Preece or modified Preece tests withacidified copper sulfate solution.

As previously indicated the composition of the emulsion to be applied asa coating may vary somewhat depending upon the nature of the substrate.Thus, in applying the emulsion coating compositions to aluminum (thisterm also includes aluminum alloys) experience has shown that excellentresults are obtained either with high or low hexavalent chromiumcontents and either at low pH's or high pH's, although the pH ispreferably within the range from 5.8 to 10.5. On the other hand, whenthe emulsion coating compositions are applied to steel, the pH should bebelow 7.0. In the treatment of galvanized iron or steel (i.e., zincsurfaced articles), it is preferable to have the hexavalent chromiumpresent in amounts such that a coating of the emulsion coatingcomposition on the substrate contains not more than 10 micrograms persquare inch of hexavalent chromium, as Cr, and the pH is preferably 6.8to 8.0 for best results. If too much hexavalent and active chromium ispresent it reacts with the zinc forming zinc chromates which tend toform a powder and give poor adhesion where a finishing top coat isapplied.

The application of the emulsion coating compositions to magnesium (whichincludes magnesium alloys) is similar to that for aluminum althoughmagnesium is very sensitive to low pH values below 5.0.

The invention will be further illustrated but is not limited by thefollowing examples in which the quantities are given in parts by weightunless otherwise indicated.

EXAMPLE I

An emulsion coating composition was prepared as follows:

Four gallons of an acrylic emulsion polymer containing 46±0.5% solidsand having a pH of 9 to 11 (Rhoplex MV-1) was mixed with a premix of 15to 30 ml of tributyl phosphate and 3 to 10 ml of a defoaming agent(NOPCO NXZ) and agitated thoroughly until no fish eyes or agglomeratesappeared on a fineness of grind gauge. 1 to 2 gallons of water was addedand to the resultant emulsion while it was being rotated with apaddle-type agitator there was added dropwise at the periphery of theemulsion body a dilute solution of sodium dichromate in water containingone gram of sodium dichromate per 10 ml of solution until a pH value of6.6 to 6.8 was obtained. The resultant emulsion was then filtered.

A coating of the foregoing emulsion was applied using a draw bar to athickness of 0.05 mil to 1.0 mil thickness on aluminum panels of No.3003 alloy having a gauge thickness of 0.019 to 0.025. These panels hadpreviously been alkaline cleaned. Some were chromate conversion coated.Others were rinsed with water and then acid rinsed with chromicacid-phosphoric acid mixtures and dried prior to the application of thecoating.

The coatings were cured by preheating at 120° F. to 160° F. for 20 to 40seconds followed by complete cure in 45 to 60 seconds at 600° F. Panelsof the cured coatings were bent and the edge of the bend exposed to 60inch-lbs. direct impact. There was no "pull away" using 3M-600 tape.

EXAMPLE II

A coating composition was prepared by mixing 20 ml of an acrylicemulsion polymer of the type described in Example I with 20 ml of waterand adding dilute sodium dichromate dropwise in the manner described inExample I until the pH was 6.6 and thereafter adding aqueous ammoniumhydroxide (28%) to a pH of 9.0 to 9.5.

To this composition there was then added 60 ml of a thermosettingacrylic emulsion polymer (Rhoplex AC-604) having a solids content of46±0.5% and a pH of 9.5 to 10.5. To this composition there was alsoadded 10 ml of an epoxy resin (Ciba-Geigy Pro-808) having a pH of 7.5and a premix of 5 ml of tributyl phosphate and 1 ml of a defoaming agent(NOPCO NXZ). In addition 1 ml of propylene glycol and 3 ml of n-butylalcohol were added and the composition was mixed until clear of all fisheyes and agglomerates.

The foregoing composition was applied to a hot dip galvanized 0.025gauge metal panel which had previously been cleaned and prepared. A drawbar was used to apply a coating of 0.2 mil wet film.

The coated panel was then preheated to 160° F. for 30 seconds and curedat 600° F. for 75 seconds. It had a pencil hardness of 2H and after 60minutes in boiling water with a 15 minutes recovery time the pencilhardness was 2H/H.

Impact tests on the coated panel were carried out with excellentresults.

EXAMPLE III

A pigment was prepared by mixing together in water 1 pound of slakedlime (CaOH)₂, 1 liter of water and 15 ml of a surfactant (Triton CF-10)to form a paste, 26% fluosilicic acid was then slowly added withthorough mixing until a pH of 8.0 was obtained. The reaction was highlyexothermic and the mixer was water cooled. Thereafter, chromic acid wasadded until the pH was 7.0. This gave a pigment containing hexavalentchromium. If the addition of chromium is not desired, another acid suchas phosphoric is used to neutralize the composition to the desired pH of7.0, immediately before using-- this is a catalyst addition. The pHvalue can be altered.

In either case the resultant composition was then ground in a ball milland about 1/2 pound of synthetic talc was used to reduce gloss.

One to two pounds of titanium dioxide were then added (duPont 966 TiO₂).A premix was prepared by mixing together 500 ml of tributyl phosphate,100 ml of a defoaming agent (NOPCO NXZ), 1 gallon of propylene glycoland 75 grams of a pigment dispersant and stabilizer (Tamol 830) added tothe foregoing composition. The resultant mixture was milled until nofish eyes, agglomerates or large particles appeared on a ground gaugeand the particles had a size within the range of 0.5 to 5 microns.

1000 ml of thermoplastic acrylic emulsion polymer (Rhoplex MV-1) wasthen added and milled with the pigment until all of the pigmentparticles were uniformly coated.

In a slow paddle wheel mixer 5 gallons of the aforesaid thermoplasticacrylic emulsion polymer was mixed with 2 gallons of water and thepigmented mixture slowly added thereto. Thereafter a dilute solution ofsodium dichromate containing 1% to 10% by weight hexavalent chromium, asCr, was added dropwise to the peripheral portion of the rotating mixtureuntil a pH of 7.6 to 7.8 had been obtained.

A diluted thickening agent was then added (Acrysol G-110) until aviscosity reading of 25/30 seconds was obtained using a Zahn No. 2 cup.2 gallons of an acrylic thermosetting polymer (Rhoplex AC-604) were thenadded and the mixture was mixed slowly for at least 6 hours.

A coating of the foregoing emulsion was applied to both 3003 aluminumsheet and spangle hot dip galvanized sheet to a wet thickness of 0.2 milusing a draw bar. The coated hot dip galvanized sheet was preheated to160° F. for 30 seconds and cured at 600° F. for 75 seconds. The coatedaluminum sheet was preheated at 160° F. for 30 seconds and cured at 600°F. for 60 seconds. In each case the resultant coatings had a pencilhardness of 2H/H.

The hot dip galvanized coated sheet withstood the impact of 80 to100"-pounds and the coated aluminum sheet withstood impact of30"-pounds. The aluminum sheet after being bent withstood an edge impactof 60"-pounds. The impact value (lbs/sq. in.) is that just before metalor crystal fracture occurs, thus, this value must be a variable.

Salt spray tests on both sheets were conducted to failure. The coatedhot dip galvanized sheet ran 400 to 600 hours and the coated aluminumsheet ran over 1000 hours. On embossed aluminum sheeting a coating ofthe foregoing emulsion applied as previously described gave verysuperior results on sharply rounded areas as tested by the modifiedPreece copper sulfate test.

EXAMPLE IV

20 grams of lead-silicate-chromate pigment (ONCOR-M50), 10 grams ofstrontium chromate, 20 grams of water and 12 ml of a dispersant (Tamol850) were ball mill ground to a particle size of 0.5 to 5 microns. 100ml of acrylic polymer (MV-1), together with a premix of 5 ml of tributylphosphate and 1 ml of a defoaming agent (NOPCO NXZ) were added and thegrinding continued until all of the ingredients were uniformlydispersed. There was then added 100 ml of acrylic polymer (MV-1), 100 mlof acrylic thermosetting polymer (AC-604) and 100 ml of water andgrinding continued for 30 minutes. The liquid mixture was then subjectedto a slow speed agitator (1-60 rpm) and a dilute solution of sodiumdichromate (1 gram in 10 grams of water) was added slowly to a pH of6.8.

Using an 0.2 mil draw bar, coatings of the foregoing composition wereapplied to low carbon steel panels which had previously been cleaned andprepared and the coated panels were preheated at 160° F. for 30 to 45seconds, then cured at 600° F. for 60 to 75 seconds.

The resultant coated panels withstood an impact of 160"/lbs. with notape pullaway, a bend test with no visible fracture or tape pullaway anda salt spray test using a 5% sodium chloride mist with an exposure of500 hours before signs of blister and corrosion, and no tape pullawayafter 15 minutes recovery time.

EXAMPLE V

50 grams of strontium chromate, 50 grams of water and 13 ml ofdispersant (Tamol 850) were mixed with a premix of 2 ml tribtuylphosphate, 1 ml surfactant (Triton CF10), and 1 ml defoaming agent(NOPCO NXZ) and ball milled for 4 hours. 500 ml of thermoplastic acrylicemulsion polymer (RHOPLEX MV-1) was added to the mixture and ball milledfor 30 minutes.

The resultant mixture was then subjected to slow agitation and 1000 mlof said thermoplastic acrylic polymer (MV-1) and 500 ml of an acrylicthermosetting polymer (AC-604) were added. While the resultant emulsionwas being agitated slowly a dilute aqueous solution of sodium dichromate(1 gram in 10 ml of water) was slowly added at the periphery of therotating body until a pH of 8.4 was obtained.

The resultant emulsion coating composition was then applied to properlycleaned and surface prepared spangle hot dip galvanized panels toproduce a wet coating having a thickness of 0.2 ml using a 0.2 mil drawbar. The coated panels were preheated at 160° F. for 30 seconds, curedat 600° F. for 60 seconds, 90 seconds and 120 seconds respectively. Theywere then subjected to indirect impact tests at 120"-lbs. and showed nopullaway but with some fracturing of galvanize crystals appearing. Apencil hardness test gave a 3H to 2H to all panels.

All of the panels were then placed in boiling water for one hour andtested for pencil hardness after a 15 minute recovery period. The panelwith the 60 second cure softened badly and absorbed water. The panelwith the 90 second cure softened somewhat and the panel with the 120second cure recovered with very little change as compared with the panelbefore the boiling water test. The 120 second cure panels when subjectedto indirect impact testing at 120"-lbs. gave no tape pullaway.

Coated panels prepared as previously described will adhere to finishingcoats including, for example, polyester (e.g., PPG-JJ-487 Duracron),acrylic (e.g., PPG 11W30, Duracron 100, duPont 876-5461, 876-559 and876-5484), polyvinyl chloride (e.g., Sherwin-Williams G-77WC198),modified silicone (e.g., 64X423), modified polyester (e.g., Dexstar5X100A), modified epoxy (e.g., Dexstar 9X165), and aqueous resincoatings (e.g., Armorcote 11 White).

Since all of the chromate-type primers have a decided color, whenever atop coating is to be white or pastel in color, there is produced adecided color detraction after the baking operation. To offset thiseffect rutile titanium dioxide is incorporated and sometimes theaddition of a blue pigment dispersible in water (e.g., Hercules ImperialA-984, or X2925) to improve the final color effect for whites, blues andgreens. For other shades some comparable color pigments akin to thedesired color tone can be added. These tend to block out or hide thecolor effect of the active chromates contained in the prime coating.

As will be apparent from the foregoing description, the emulsion coatingcompositions prepared in accordance with the invention can contain awide variety of ingredients in various proportions. In general, however,these compositions fall into two main categories depending upon whetherthe resinous binder is a thermoplastic resin or a combination ofthermoplastic and thermosetting resins. Where the resinous binder issolely a thermoplastic resin, the composition of the emulsion ispreferably as follows:

    ______________________________________                                        Thermoplastic resin (e.g., Rhoplex MV-1)                                                            50-95% (Vol.)                                           Water                 45-0%                                                   Dilute inorganic chromate solution (e.g.,                                      Na.sub.2 Cr.sub.2 O.sub.7)                                                                         Enough to give a pH                                                           of 5.5-9.0                                              Pigment (e.g., SrCrO.sub.4 or CaSiF.sub.6, or                                  chromate containing CaSiF.sub.6)                                                                   0-1% (wt.)                                              Coalescing agents (e.g.,                                                       tributyl phosphate)  0.5-3% of the resin                                                            (Vol.)                                                 Surfactant (Triton CF10)                                                                            Trace to 0.1% of                                                               resin                                                  Thickening Agent (Acrysol G-110)                                                                    Trace to 0.5% of                                                               resin                                                  Titanium dioxide      0-5% (wt.)                                              Dispersing agent (e.g., Tamol 850)                                                                  0-0.5% of pigment                                                              (wt.)                                                  In the case of a composition containing both thermoplastic                    and thermosetting resins a preferred composition contains:                    Thermoplastic resin (e.g., Rhoplex MV-1)                                                            15-60% (vol.)                                           Thermosetting resin (e.g.,                                                     Rhoplex AC-604       10-60% (vol.)                                           Water                 10-4%                                                   Dilute chromate solution (e.g., Na.sub.2 Cr.sub.2 O.sub.7)                                          Enough to give pH                                                             of 6.8-9.5                                              Pigment (e.g., SrCrO.sub.4, CaSiF.sub.6 or                                     chromate containing CaSiF.sub.6)                                                                   0-1% (wt.)                                              Coalescing agents (e.g.,                                                       tributyl phosphate)  0.5-3% of the resin                                                            (vol.)                                                 Surfactant (Triton CF10)                                                                            Trace to 0.1% of                                                               resin                                                  Thickening Agent (Acrysol G-110)                                                                    Trace to 0.5% of                                                               resin                                                  Titanium dioxide      0-5% (wt.)                                              Dispersing agent (e.g., Tamol 850)                                                                  0-0.5% of pigment                                                              (wt.)                                                  ______________________________________                                    

In these compositions ammonium hydroxide, dimethylethanolamine, ordiethanolamine are added with or without the thickener to obtain adesired viscosity which is within the range of 20 to 45 seconds on aZahn No. 1 cup and 17 to 35 seconds on a Zahn No. 2 cup. The method ofapplication and desired dry-film thickness will determine the viscosityvalue, along with the equipment for curing such applied film.

Certain catalysts can be incorporated to: increase the speed of cure,and/or lower the "cure" temperature. Such materials are, for example,thiocyanates (provided iron is not present to cause discoloration),toluene sulfonic acid, and phosphoric acid. It is suggested that if suchbe used, this along with other additives such as water, for viscositycontrol, should be added just prior to mixing (at usage station).

Also, it is desirable to keep the organic film-coating mixture at a pHvalue of 8.0 to 8.5 and make the final adjustment along with theaforementioned additions. This affords a more accurate pH control forwhatever the desired pH value may be.

In practicing the invention many different methods of adding dilutesolutions of the water soluble chromium containing compounds were triedincluding (1) injection using a pump into the middle of the liquid body;(2) injection using a high volume circulating pump by drawing from thebottom of the tank and then discharging into the middle of the liquidbody or onto the liquid surface; (3) pouring the dilute solution in thevortex produced by a high speed mixer; (4) pouring powdered chromiumcontaining material into the vortex of the liquid body (these gave verypoor results); (5) adding approximate amounts of the dry chromiumcontaining compound to water to be used and then adding the resinousemulsion; (6) adding both solid chromium containing compound and dilutedsolution separately into the final adjusted liquid mix; and (7) sprayingdiluted chromium containing solution onto and into the rotating liquidmixture containing the resin. None of these methods gave results as goodas the preferred method described herein wherein a dilute aqueoussolution of the chromium compound was added gradually to a peripheralportion of the rotating body.

The success of the process appears to depend upon a combination of thefactors previously outlined, namely, the gradual addition of a diluteaqueous solution of an inorganic ionizable water soluble hexavalentchromium compound having a concentration of 1 to 10% by weight,calculated as Cr, cooling so that the temperature does not exceed 90°F., maintaining a pH within 2 to 10.5 and maintaining the body ofemulsion in a non-foaming state. The use of these conditions reducesesterification and aldehyde formation and the amount of sediment formedin the process. In the final analysis the amount of sediment formed is ameasure of the success of the process. Under the worst conditions largeamounts are formed whereas under the conditions employed herein it ispossible to practice the process so that no more than 25 to 35 grams ofsediment is produced in a 55 gallon batch.

The invention is hereby claimed as follows:
 1. A process of protectingmetals which comprises coating said metals with an emulsion coatingcomposition containing water in the continuous phase and having in thediscontinuous phase at least one water insoluble particulatefilm-forming thermoplastic resin and a water insoluble pigment selectedfrom the group consisting of strontium chromate, calcium silicofluorideand a chromate containing calcium silicofluoride, the weight ratio ofsaid pigment to said resin being within the range of 1:99 to 40:60, thesolids content of said composition being within the range of 15-35% byweight, excluding pigmentation, said composition also containing asubstance from the group consisting of ammonium hydroxide,dimethylethanolamine and diethanolamine, the viscosity of saidcomposition being within the range of 25-45 seconds on a Zahn #1 cup and17-35 seconds on a Zahn #2 cup, and thereafter removing water from saidcomposition, the amount of said composition applied to the metal beingsufficient to produce a uniform coating having a wet thickness withinthe range of 0.05 mil to 10 mils.
 2. A process as claimed in claim 1 inwhich the thickness of the wet coating is within the range of 0.1 to 0.3mil.
 3. A process as claimed in claim 1 in which the metal to which thecoating composition is applied is steel and the pH of the coatingcomposition is less than
 7. 4. A process as claimed in claim 1 in whichthe metal to which the coating composition is applied is galvanizediron, the pH of the composition is at least 7 and the quantity ofhexavalent chromium, as Cr, applied to the metal does not exceed 10micrograms per square inch of metal.
 5. A process as claimed in claim 1in which the metal to which the coating composition is applied isaluminum.
 6. A process as claimed in claim 1 in which said emulsioncoating composition is alkaline and the alkalinity is attributable atleast in part to a compound from the group consisting of ammoniumhydroxide, diethanolamine and dimethylethanolamine.
 7. A process asclaimed in claim 1 in which water is removed from the wet coating of thecoating composition and the composition is cured after it has beenapplied to the metal by heating to temperatures within the range of 400°to 600° F. for periods of from 30 seconds to 3 minutes, the time ofheating being longer at the lower temperatures and shorter at the highertemperatures and being sufficient to form a coating having a pencilhardness within the range of H to 2H.
 8. A process as claimed in claim 1in which said coating composition contains a pigment consistingessentially of CaSiF₆ and 0% to 10% by weight, calculated as Cr, ofhexavalent chromium added as an inorganic ionizable water solublehexavalent chromium compound.
 9. A metal coated with a coatingcomposition as claimed in claim
 1. 10. A metal coated with a coatingcomposition as claimed in claim 8.